1. Field of the Invention
This invention relates generally to corrosion monitoring and measuring apparatuses and more particularly, it relates to a method and apparatus for monitoring reliably and realistically the localized corrosion rate of metals exposed to an aerated electrolytically conductive liquid environment. In particular, the present invention is directed to a method and apparatus for measuring underdeposit corrosion rate or metal corrosion rate under tubercles in cooling water systems.
2. Description of the Prior Art
As is generally known in industrial and commercial systems having a metal part which comes in contact with an electrolytic fluid, a major problem is localized corrosion, for instance pitting or crevice attack, because such corrosion will occur intensely in one particular location and may cause perforations in the structural member carrying the electrolytic fluid. Obviously, these perforations may cause leaks which require shutting down of the entire industrial system so that repairs can be made. As a result, the useful life of the structural member will be shortened. Pitting/localized corrosion is typically far more of a concern than general corrosion since general corrosion occurs essentially at a constant rate over an entire surface and will not cause a potentially dangerous leak in a short period of time as in the case of localized corrosion.
Therefore, there has arisen a need for corrosion monitoring systems for the purpose of estimating the residual service life of the structural member. Further, in industrial systems for cooling water treatment application there is an urgent demand for monitoring effectively and realistically the performance of a given chemical treatment program for controlling localized corrosion.
There are known in the prior art various corrosion monitoring apparatuses for determining general and/or localized corrosion rates. For example, in U.S. Pat. No. 3,660,249 issued on May 2, 1972, to C. R. Townsend, there is disclosed a method and apparatus for determining both the general or average corrosion rate and the pitting tendency of a metal exposed to an electrolyte which utilizes a corroding electrode, a reference electrode, and an auxiliary electrode. The corroding electrode is made 10 mv positive relative to the reference electrode and is then made 10 mv negative relative thereto. The anodic and cathodic currents which flow through a circuit including the corroding electrode and the auxiliary electrode are then averaged to produce an indication of the general or average corrosion rate. The difference between such anodic and cathodic currents is determined in order to provide the pitting tendency or pitting index of the system.
In U.S. Pat. No. 3,878,064 issued on Apr. 15, 1975, to A. Weisstuch and C. E. Schell III, there is disclosed a method and apparatus for measuring pitting corrosion tendencies of metals exposed to an electrolytic liquid which has a Cathode, an Anode, a Reference Electrode, and an Auxiliary Electrode positioned in the electrolytic liquid. The open circuit potential between the Cathode and Reference Electrode is impressed upon a capacitor. By means of a two-position switch, an operational amplifier then causes the potential of the Anode, in a circuit including the Reference Electrode, Auxiliary Electrode (or the electrode previously used as the Cathode) and an Ammeter, to be equal to the open circuit cathode-reference electrode potential. The amplifier provides the necessary current of either polarity so as to achieve this equality. The polarization current flowing through the Ammeter is read and a qualitative indication of the pitting tendency is obtained.
U.S. Pat. No. 4,575,678 issued on Mar. 11, 1986, to K. Hladky teaches a corrosion monitoring apparatus for monitoring the corrosion of a metal part which includes the part defining a first electrode in contact with an electrolyte and a second electrode in contact with the electrolyte but electrically insulated from the first electrode. A high input impedance voltmeter is connected across the first and second electrodes. The low frequency voltage between the electrodes is observed. This voltage is a low frequency noise signal. The amplitude values of the signal are measured and subjected to an averaging computation yielding data indicating the corrosion rate of the first electrode and the nature of the corrosion.
U.S Pat. No. 5,045,775 issued on Sept. 3, 1991, to M. L. White and H. Leidheiser, Jr. teaches a system for monitoring and measuring the corrosion reaction of metals in an environment. The system includes a sample corrosion element of substantially the same material as the structure to be monitored, a galvanic cell for generating an electric signal indicative of the corrosion on the corrosion element, and a monitor for receiving and storing the generated signal. The system is designed to incorporate corrosion products and environmental contaminants in order to simulate actual, localized conditions on a particular area of a civil engineering structure exposed to atmospheric corrosion.
PCT application No. WO 87/07022 filed May 11, 1987, to D. A. Eden et al. describes a method and apparatus for detecting and measuring localized corrosion of a metal surface which includes an array of electrodes fabricated from the same material as the metallic surface and exposed to the same corrosion conditions as the metallic surface. The coupling current between the array of electrodes is measured, and the electrochemical current noise originating in the electrode array is measured. The two measurements are compared to provide an output indicative of the degree to which corrosion is localized. Further, the apparatus also includes means for providing a resistive/impedance noise related output based upon an electrochemical potential noise and the electrochemical current noise. The output indicative of the degree to which corrosion is localized and the resistive/impedance noise output are compared to provide a second output indicative of the rate of localized corrosion.
However, all of the prior art corrosion monitoring systems discussed have their drawbacks and none of them can provide realistic and effective monitoring of localized corrosion. It has been realized by previous experience that it is of critical importance to have the capability of monitoring both the initialization and the propagation stages of localized corrosion. Further, earlier studies have established that all of the cooling water treatment programs (i.e., stabilized phosphate, zinc containing programs, and alkaline phosphate programs) achieve their corrosion protection function by forming a compact layer of inhibitors on the metal surface in the formation process and thus the protective quality of the film depends on fluid dynamic conditions.
By realizing that the existence of different fluid dynamic conditions is an inherent property of any actual cooling water system, the inventor has developed a method and apparatus for measuring underdeposit localized corrosion rate of carbon steel and cast iron structures under differential flow conditions encountered in actual cooling water systems. This apparatus allows for the capability of evaluating a chemical treatment program against the initialization and the propagation of underdeposit localized corrosion.